The invention relates to an improved zeolite structure and a method for making such structure. In particular, it relates to a composition for such structures using solvent-free vehicle, and a method for forming cellular zeolite structures such as extruded honeycomb monoliths from such compositions.
Unlike high surface area oxides such as alumina, titania, spinel, zirconia, silica and the like, which have been known in the an for many years for being readily extrudable into high strength support such as honeycomb structures used for filters and for catalyst supports, zeolites have not been readily available in extruded form. This is due to some significant differences between zeolites and high surface area oxides which make zeolites difficult to form into large complex shapes such as honeycombs.
Recently it has been disclosed that significantly higher strengths can be obtained by incorporating permanent binders such metal oxides (silica, titania, alumina etc.) in their dry state into such high surface area batches. It has also been disclosed that even higher strengths can be achieved in such systems if the permanent binder is incorporated in the form of a precursor of the permanent binders.
More recently, it has been disclosed in co-assigned U.S. Ser. No. 08/304,157, that zeolite structures can be made with the use of silicone resin as a permanent binder. As disclosed in the reference, except for silica, precursors of the permanent binder can be used in the form of a dispersion, suspension, or solution in a liquid diluent. When the precursor of the permanent binder is silicone resin, the resin is dissolved in a solvent or a mixed solvent system of alcohol and water. The silicone resin can be mixed directly with porous oxide powders, in which case a solvent is used during the plasticizing step to dissolve the resin. Alternatively, the silicone resin can be predissolved in an appropriate organic solvent such as methyl alcohol, ethyl alcohol and isopropyl alcohol. Whether the silicone resin is predissolved in a solvent or mixed directly with the porous oxide powders, the resins are preferably milled first to a particle size finer than 20 microns, and more preferably, finer than 10 microns.
There are problems associated with solvent-based systems such as OSHA concerns (toxicity and flammability) due to the presence of strong organic solvent vapor during mixing and extrusion. There are also other concerns such as low yield. Volatile solvents tend to cause blistering and cracking under normal dielectric drying of such structures due to a rapid forming of dry skin over the substrates. To minimize the occurrence of such problems, the structures are generally dried at very slow rates in a ventilating hood over a period of several days. The slow drying is due to the tendency of such structures to rapidly form a dry skin over the substrate from the volatile substrate which also causes blistering and cracking when such structures are subjected to dielectric drying. As a result, structures formed from solvent-containing batches require slow drying in a ventilating hood, typically over a period of days to minimize the occurrence of blisters and cracks.
While and co-assigned application, U.S. Ser. No. 08/304,157 offers significant improvement over the previously known compositions and methods of forming cellular zeolite structures, further improvements can be made. Accordingly, it is a principal objective of the present invention to provide an improved composition and method for making cellular zeolite structures.